Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $91.81   8.583%   $99.6901   $99.69   $99.70   $99.70  
  Purchase #2 $5.71   12.266%   $6.4104   $6.41   $6.40   $6.40  
  Purchase #3 $21.51   8.764%   $23.3951   $23.40   $23.40   $23.40  
  Purchase #4 $206.76   10.625%   $228.7283   $228.73   $228.75   $228.70  
  Purchase #5 $223.88   7.212%   $240.0262   $240.03   $240.05   $240.00  
  Purchase #6 $33.66   8.749%   $36.6049   $36.60   $36.60   $36.60  
  Purchase #7 $8.48   3.503%   $8.7771   $8.78   $8.80   $8.80  
  7 purchase totals = $591.81       $643.6321   $643.64   $643.70   $643.60